The invention pertains generally to superconductors and in particular to superconductors made from a solid state reaction between two alloys.
In recent years superconductive materials have been proposed for use in many diversified applications. These include power transmission, levitated electromagnetic land transportation systems, naval propulsion systems, aircraft ac generators, and large laboratory research magnets. Superconducting generators may be only one-tenth to one-third the size of conventional electrical generators of the same power rating. Savings are realized in operating these systems when superconductive materials with high critical temperature, critical current density, and critical field are used.
Intermetallic compounds having an A-15 crystal structure are known to be exceptional superconducting materials. This structure is also referred to as a beta-tungsten crystalline structure. One of the ways in which these compounds are obtained is by a solid state reaction between two alloys in a vacuum or inert atmosphere at an elevated temperature. These compounds are then used in a composite structure with the two reactant alloys, often one being in the form of one or more filaments embedded in the other alloy (often referred to as the matrix). Excellent examples of such superconductors are composites of VGa-V.sub.3 Ga-CuGa, VGa-V.sub.3 Ga-CuGaAl, NbSn-Nb.sub.3 Sn-CuSn, and VSi-V,Si-CuSi. These superconductors, along with their processing, have been disclosed in U.S. Pat. Nos. 3,811,185; 3,926,684; 3,989,475; 4,002,504; and 4,190,701.
Two major objectives in research concerning these superconductors are improving the electrical properties and the processing capabilities of the materials. The critical current density and the ductility are of particular interest.
Critical current values indicate the ability of the material to carry large currents, and is defined as the maximum current passed through a conductor in a transverse magnetic field before a measurable voltage appears in the conductor. By dividing the critical current by the cross-sectional area, values for the critical current density are obtained.
It is known that reducing the grain size of the intermetallic A-15 material increases the number of flux-pinning sites and thus the critical current. One method of refining grain size is lowering the A-15 formation temperature which slows down the solid state reaction and increases production costs. Varying the composition of the reactant alloys in the composite can improve the grain size, but can also detrimentally affect the processing of the composites.
Processing the composites into useable shapes is extremely difficult and is often just barely possible. The reactant alloys become hard and brittle through work hardening and the A-15 compounds are inherently brittle.
Much of the work hardening occurs because of interstitial impurities. Another problem associated with impurities in the reactant alloys is the adverse affect impurities may have on the tightness of the bond between the reactant alloys. Although the processing technique disclosed in U.S. Pat. No. 4,002,504 by Howe has produced a superconductor with few interstitial impurities, the amount of impurities still present causes serious problems.